In this article we will discuss about Humans:- 1. Evolutionary Trends of Humans 2. Closest Relatives of Humans 3. Ancestry 4. Modern Human Beings 5. Classification.
Contents:
- Evolutionary Trends of Humans
- Closest Relatives of Humans
- Ancestry of Humans [Early Evolution of Apes and Hominids]
- Modern Human Beings
- Classifications of Humans
1. Evolutionary Trends of Humans:
Numerous studies have been made to ascertain the origin and evolution of humans. The fossils which have been so far unearthed indicate that man has originated somewhere in the plains of Asia, Africa and Europe from some non-human beings, similar to proconsul — the prehistoric African anthropoid apes — in late Miocene period.
The exact course of evolution is still not known due to paucity of fossil records. The following steps are being discussed that help in tracing out the human ancestry and the evolution of humans (Homo sapiens).
a. Bipedal locomotion with erect posture, straightening knees, flattened feet, and shortening of toes.
b. Perfect erect posture is due to the development of S-shaped curvature in the vertebral column.
c. Broadening of iliac bones of pelvic girdles and position of acetabulum.
d. Freeing of forelimbs from ground for manipulation. The forelimbs with opposable pollex (thumb) are used in grasping, and other mechanical functions.
e. Loss of hairs over the body except sole, palm and parts of facial region.
f. Orthognathous face (vertical or vertically placed skull).
g. Reduction in size of canines and incisors.
h. There is a reduction of jaw with smaller chewing apparatus.
i. Dental arch is smooth and semicircular.
j. Reduction in size of eyebrow ridges and simian shelf.
k. Loss of opposability of great toe (hallux) in hind limb.
l. Development of chin.
m. Increase in cranial capacity, 1390 cc with brain to body weight ratio being highest 1: 46 as compared to 1: 250 in apes. The average size of the brain of human beings is the highest as compared with the brains of apes and human beings.
n. Reduction of nozzles.
o. Loss of jaw power.
p. Development of articulate speech and sharp memory.
Pre-primate ancestry:
The sequences are as follows:
Sarcopterigian fishes (Lobe-finned fishes) —> Labyrinthodont amphibians —> Cotylosaur reptiles —> Therapsid reptiles —> Primitive mammals —> Jurassic pantotheres —> Mammals of Order Insectivora.
Primate ancestry:
The earliest primates appeared among the late Cretaceous under the order of placental mammals, differing probably only a little with the insectivores from which they arose.
2. Closest Relatives of Humans:
Among primates the Chimpanzee and Gorilla are judged to be man’s closest relatives on the basis of their structural, serological, biochemical as well as chromosomal and molecular level of similarities.
A. Structural similarities:
(i) More or less bipedal locomotion,
(ii) Semi-erect or erect posture,
(iii) Absence of tail in both cases,
(iv) Fewer lumbar vertebrae,
(v) Presence of brow ridges,
(vi) Grasping pollex in both cases,
(vii) Menstrual cycles in females.
B. Serological similarity:
AB blood group is found in both apes and humans.
C. Biochemical similarities:
(i) Haemoglobin analysis between chimpanzees and humans indicates that one amino acid is different in between two forms,
(ii) Serum tests indicate protein homology to be maximum in between humans and chimpanzee, then less maximum between gorilla and humans and, at last less similarly between the humans and Asiatic apes [Holoock (Hylobates), and Orangutang (Pongo)].
D. Cytological and Molecular homology:
The fossil record of living African apes is not known. For comparison in between African apes and human beings we must rely on cytological and molecular studies. From molecular studies it is known that gorillas were diverged from the Chimpanzee and human beings from a common ancestor about 8-10 million years ago. Again, Chimpanzee and humans were isolated about 6 million years ago.
At present the differences are:
(i) DNA content in diploid cells is similar,
(ii) Hybridization of non-repeated DNA sequences of chimpanzees and humans indicate 98% similarity,
(iii) Banding pattern between the chromosomes of apes and humans shows that maximum chromosomes of humans are similar with apes except 3 and 6 chromosomes where little differences occur,
(iv) Human beings have 46 chromosomes while apes have 48.
Differences between apes (Pongidae) and Humans (Hominidae) (Fig. 10.83):
Due to anatomical differences between apes and humans, they are placed in different families.
The differences are:
(i) Humans have an erect, bipedal posture, and it is due to the “S”-shaped curvature of the vertebral column and broadened pelvis, and position of the acetabulum. In apes, the posture is semi-erect and pelvis is elongated,
(ii) The jaws of apes are either rectangular or “U” shaped but in human beings, it is bow-shaped,
(iii) In apes larger incisors, “U”-shaped dental arch and lower premolars with cutting edges. In humans the dental arch is a smoothly rounded parabola and small incisors,
(iv) There is a simian gap among apes in between incisors and canine on each side of the jaw. Simian gap or diastema is absent in humans,
(v) The palate of the humans is arched but in apes it is flatter between the parallel rows of cheek teeth,
(vi) As compared with human beings, the forelimbs of apes are longer but, in human beings, the legs are larger and stronger than arms,
(vii) The body of apes has a thick growth of hair but, in humans body hair is much reduced and development of large number of sweat glands may be correlated with this,
(ix) The skull of gorilla (Fig. 10.84A) possesses brow ridges and large sagittal and nuchal crests which increase the area and help the attachment of jaw muscles. In humans such crests are not found,
(x) The ape’s brain differs in size and organization from the human’s. In human brain parietal, frontal and temporal areas of the cortex are enlarged and more convoluted than the ape’s brain. Apes are less intelligent, with less cranial capacity as compared with humans. Cranial capacity of Orangutan and Chimpanzee is 400 cc, and 550 cc in Gorilla, whereas in humans it is in between 1400 – 1450 cc.
(xi) In humans, neck is long and the position of foramen magnum is at the rear of the brain-case. In apes, the neck is short and the position of the foramen magnum is more ventral than humans.
3. Ancestry of Humans [Early Evolution of Apes and Hominids]:
There is no clear-cut idea about the ancestry of Hominidae. Different opinions prevail regarding the origin of Hominidae.
According to one group the earliest known ape is Aegyptopithecus which had a small brain, a tail and feet and hands that indicate an arboreal quadrupedal life. A robust ulna also indicates that they were canopy dweller. By Miocene time it could have led to the genus Dryopithecus (subfamily Dryopithecinae).
As early as middle Miocene times, 15 to 25 million years ago, Dryopithecus (= Proconsul) led an arboreal life and lived in Africa, Europe and Asia. Fossils of Aegyptopithecus were collected from Fayum in Egypt and Dryopithecus from Kenya. The skull of Dryopithecus was lightly built than that of modern man. There were no brow ridges.
Small incisors were like those of modern man. Canines were large. The molars had 5 cusps, arranged in Y-pattern. They were probably the common ancestors of the present day ape (Pongidae) and humans (Hominidae) (Fig. 10.85).
According to another group of scientists, the Eocene prosimians (Lemurs and Tarsiers) gave rise to Oligocene ancestors of Old World Primates, i.e., Parapithecus. It was a squirrel- sized creature, known from a single lower jaw and upper jaw fragments, found at Fayum in Egypt, and could have been the ancestors both of Old World monkeys and of apes (Fig. 10.85).
Earliest Hominids:
In late Miocene and early Pliocene, 14-8 million years ago, there lived in the Siwalik Hills of India and Pakistan, Ramapithecus (= Shivapithecus). Related fossils from Maboko Island and Fort Ternan in Kenya, Hungary and Greece are Kenyapithecus.
They are closer than any other known ape-like forms to the ancestry of Hominidae. Their fossil features indicated that they walked on legs and dentition was similar to modern man. The fossil features also suggest that they lived at the side of the forests and on the open lands.
Ardipithecus Ramidus:
Recently some fragments with four teeth of a new species of fossil hominid, as old as 4.4 million years old, were collected from the sediments of Ethiopia and named Ardipithecus ramidus (formerly called Australopithecus ramidus).
The features of A. ramidus indicate that they were bipedal, had human-like arms and incisiform canines. By judging the features it is assumed that they were the first ape-like hominid ancestor known.
Southern Ape Men:
Australopithecus [L. australis = south + pithecus = ape] (Fig. 10.84):
In evolution the next primate that followed Ramapithecus was Australopithecus that was first found in South Africa in 1924 by R. Dart, and are now almost unanimously accepted as links between the ape-like and human stages of our ancestry.
They lived from at least 3 million to perhaps 1 million years ago. Most of the known remains have been found in southern and east central Africa. There existed at least three species. These three species — of which none represents our ancestors — evolved parallel to them.
Of the three species—a larger with sagittal crests on the skull and less man-like called Australopithecus robustus, not generally considered in direct Homo ancestry, and a smaller without sagittal crests on the skull, a more gracile, A. africanus. A third species, A. boisei, found in East Africa was originally called Zinjanthropus by husband-wife team of Louis Leaky and Mary Leaky in 1959.
Of three species the first apparently died out without issue while the second is our likely ancestor. Dobzhansky et al., (1977) consider two species instead of three.
They presume that there are two species — Australopithecus robustus (= Zinjanthropus boisei) and A africanus but Pough et al., (1996) consider that there are three robust forms — A. robustus (South Africa), A. aethiopicus and A. boisei (East Africa) and were terrestrial, savanna-dwelling vegetarian inhabitants.
The first fossil of a juvenile skull (Taung baby), collected from a cave at Taung in South Africa, was described by Prof. Reymond Dart in 1924. It was named A. africanus (Fig. 10.84B); other fossils under the same genus were collected from East Africa, in the deposits between three and one million years old. The body structure of Australopithecus is a fascinating mixture of human, ape and intermediate traits.
The most significant features of Australopithecus are as follows:
i) Small statured forms averaging up to 4 feet (1.2 m) tall.
ii) The cranial capacity was 450-500 cc, quite larger than that of chimpanzee.
iii) The shape of the pelvis indicates that these creatures walked erect.
iv) The dentition was human-like than ape-like as canines were small like those of man but simian gap was present.
v) Occipital condyles are anterior in position.
vi) Palate reduced but longer than in man.
Remarks:
By judging the above features Alexeev (1986) remarks that Australopithecus is the earliest stage in the evolution of the family Hominidae, an intermediate link that unites man proper with the animal kingdom.
A. afarensis:
Another a single young female adult skeleton was discovered by Donald Johanson in 1974 in the desert of the Afar Plain and also in Ethiopia. It was named Australopithecus afarensis but is known in the popular literature by the nickname Lucy (named after a Beatles song). It was about 3.2 million years back. Lucy was about 105 cm tall and weighed perhaps 30 kilograms.
The cranial capacity of other individuals of A. afarensis is between 380-450 cubic centimeters, quite resemble to modern chimpanzee and gorilla. Other features like pelvic girdle, molar teeth, and short finger are like humans.
The pelvic girdle and limb bones indicate bipedalism but reflect partial arboreality from her hand bone. Lucy and her relatives were either contemporary with or gave rise to other hominids such as A. africanus (Fig. 10.84B), another Pliocene form, which may be considered as the ancestor of Homo.
Homo habilis [L. homo = man + habilis = handy]:
Again, some fragments were discovered in a deposit about 2-2.5 million years ago at Koobi Fora in Kenya in 1961 by R. E. Leaky. The remains are more modern and may be near about our ancestry and it is included under the genus Homo and named Homo habilis (tool maker). The early men had perfect bipedal locomotion.
Cranial capacity is between 500-750 cubic centimeters and considered to be distinct from Australopithecus. Their height was 150 cm, weight 40-45 kg, protruded jaws and lower jaw lightly built. They had larger front teeth and smaller cheek teeth. They knew to use the chipped stone tool.
Homo Ergaster:
A more advanced skeleton of hominid, called Homo ergaster, appeared between 1.9 and 1.5 million years ago. The shape of the body and limb proportions appears as the modern human beings. Jaws and teeth are also appear like the modern man. So Homo ergaster may be considered as first hominid and safely be placed under the genus Homo.
Homo Erectus (Fig. 10.84):
As long as 1.8 million years ago, Australopithecines succeeded by large brained forms; it was named under the heading Pithecanthropus. Remains of Pithecanthropus (Homo) erectus were first unearthed in Java in 1891 by a Dutch scientist E. Dubois.
Similar fossils were in a cave near Zhoukoudian, China, about 50 kilometers south-west of Beijing, around 1920 and named Sinanthropus pekinensis. But Mayr (1950) replaced both names by Homo erectus, so that Pithecanthropus erectus is known as Homo erectus erectus (Fig. 10.84C) and Sinanthropus pekinensis as H. e. pekinensis. Similar fossils were found at Olduvai in East Africa and in Hungary.
The features of H. e. erectus were tall body, about 1.85 m in length and of weight 70 kg. The cranial capacity was between 775-1100 cubic centimeters as compared with Australopithecus— 350-750 cubic centimeters. They had prognathous (projecting) jaw, small chin, large jaws, flat nose, prominent bony eye-brow ridges, and protruded thick lips. They made stone tools and presumed to light fire.
The fossils of Peking man (H. e. pekinensis) were discovered by Davidson Black in 1920. They lived about 460000 years ago.
The features almost similar with H. e. erectus except the cranial capacity is larger than Java man ranging from 850-1300 cubic centimeters with a mean of 1075 cubic centimetre as compared with 280-500 cubic centimeters in chimpanzee, 350-750 cubic centimeters in gorillas, and H. sapiens from 950-2200 cubic centimeters.
Remarks:
The increased brain size of H. erectus may be directly related to a group mobility and intercontinental distribution and is probably the sister group of H. sapiens which came into existence about 200000 – 300000 years ago.
Evolutionary sequence:
At present the evolutionary sequence Australopithecus africanus à Australopithecus (Homo) habilis à Homo erectus à Homo sapiens seems to be the most plausible (Fig. 10.84). The transition from Homo erectus to Homo sapiens also needs clarification Dobzhansky et al., (1977). But some believe that Homo sapiens evolved from H. erectus in Africa at least 300000 years ago.
Several fragmentary remains of our earliest known species occurred at Swanscombe, in England, and Steinheim in Germany which have been dubbed “Presapiens” and lived at the last interglacial period (Wiirm) of the Ice age. These “Presapiens” fossils are thicker and more heavily built than modern skulls and may be classified in a subspecies Homo sapiens steinheimensis.
The Neanderthals, Homo neanderthalen sis (Fig. 10.84D), who inhabited Europe about 100000 years before seem to resemble modern Homo sapiens rather less than the older “Presapiens”. The first fossils of Homo neanderthalensis were discovered from the Neander Valley in Germany in 1856.
The characters of Neanderthals are more or less same except the following characters:
(i) Neanderthals were much stronger than the present humans,
(ii) Heavy head and enormously developed face,
(iii) Thick brow ridges,
(iv) Cranial capacity was 1400 cubic centimeters,
(v) Large noses and broad mid face region,
(vi) They had well developed incisor and canine teeth. Neanders adapted to severe climatic conditions and the race was replaced by a less rugged and culturally advanced H. s. sapiens.
Some 35000 to 40000 thousand years ago Neanderthals abruptly vanished in Europe, long after Homo sapiens sapiens. They developed Mousterian culture that includes stone axes, scrapers, knifes, spear points, saw-edged and notched tools used for spear handles. They had belief in the supernatural power and they buried their dead bodies with food and tools.
Cro-Magnon people (Homo sapiens fossilis) replaced the classic Neanderthals in Western Europe whose skeletal parts are indistinguishable from those of modern European man. The fossils of H. s. fossilis were found in a French rock shelter in 1868. They lived in various places in western and central Europe; throughout the Old Stone Age periods (35000 years ago).
The distinctive features of the Cro-Magnon people are:
(i) Forehead was broad,
(ii) The brain capacity was 1660 cubic centimetres,
(iii) They were 5′-6′1/4” in height.
Cro-Magnon were expert in making weapons and used clothing and fire. They began constructing their own dwellings and living in communities. They developed Aurignacian culture and had left some fine paintings in caves in France and Spain.
Remarks:
It is now believed that Neanderthals did not become extinct but were merged with modern H. sapiens sapiens on account of interbreeding and hybridization.
Humans today (Homo sapiens sapiens) [L. homo = man; L. sapere = to be wise]:
Further evolution of man after Cro-Magnon involves only cultural evolution rather than the anatomy, and superior technique of manufacturing tools are associated with time. Modern men evolved about 25000 years ago but spread to various parts of the world about 10000 – 11000 years ago.
The earliest of such skulls unearthed in Omo, Kenya about 130000 years ago. It is difficult to give the exact characteristic features of modern man because they are so varied.
Features that may be used to define subspecies are:
(i) Thinning of skull bones,
(ii) Greater breadth in the parietal region than the basal part,
(iii) Slight reduction in cranial capacity (1390 cubic centimetres)
(iv) Prominent chin (Fig. 10.84E).
Today mankind can be divided into dozens of geographic races, inhabited in the different parts of the world, sharing certain gene frequencies and traits.
The classification of mankind recognizes certain main racial groups. They are:
(i) The Mongoloids, which include Chinese, Japanese, Eskimos, Airu and American Indians;
(ii) The Caucasoids, which include the Nordic, Alpine, the Mediterranean races of Europe, the Armenoids and Dinarics of Eastern Europe, the Near East and North Africa and the Hindus of India;
(iii) The Congoids or Negroes and Pygmies of Africa;
(iv) The Capoids or bushmen and Hottentots of Africa and
(v) The Australoids, which include the Australian aborigines, Tasmanians, Negritos and Papuomelanesians.
Place of origin of man:
Early hominids:
Oldest fossil evidences indicate that ancestors of human beings appeared in Africa, mainly in East Africa about 4-5 million years ago. Also other fossils are found in China, Java and other parts of Asia.
The climatic conditions in Asia during Miocene or early Pliocene were rich due to which pre human ancestors became forced to descend from the trees and by the influence of the climatic conditions the further development proceeds.
4. Modern Human Beings:
Genetic analysis proves that the modern humans may have evolved in Africa about 0.1- 0.2 million years ago and subsequently spread in different other continents. There are two views regarding the place of origin of modern human.
1. Single Regional Hypothesis:
According to this hypothesis, modern human beings may have evolved in one continent, probably in Africa and subsequently spread to other continents and the archaic human populations were replaced by the modern humans gradually in these respective places where existed the archaic humans.
2. Multi Regional Hypothesis:
According to this hypothesis, modern human beings may have evolved simultaneously in different continents such as Asia, Africa and Europe. This view is proposed from the different fossils those have recorded in different continents. From these archaic human fossils modern human populations may have evolved in course of time.
Primate conservation:
The order primates is one of the most interesting and fascinating group in the animal kingdom as it includes 200 species of monkeys, lemurs, lorises, galagos, tarsiers and apes, and at last, the most intelligent species in the animal kingdom —the man (Homo sapiens).
With the rapid growth of Primatology over last 60 years, it has cleared the questions about our evolution and origins and they have also played a major role in biomedical research. Non-human primates are one of the major groups of animals in the world’s tropical forests and are often the best symbols for tropical forest.
Unfortunately, the wild population of the primates is decreasing all over the world. Many spectacular species like the mountain gorilla, the golden lion tamarin (Fig. 10.86), the muriqui, the indri, and the aye-aye are already on the verge of extinction and numerous other species of Asia, Africa, and Central and South America are also heading the same direction.
The major reasons for the decline of primates are the destruction of the tropical forests, the original abode of their habitat. More than 90% of the primates are living in the tropical forests of Asia, Africa, and Central and South America and these are being destroyed at the rate of 10 – 20 million hec. per year. Other reasons for decline are hunting and live capture for commercial export.
A series of major projects by the IUCN, WWF has taken to conserve the endangered primates. The highest priority regions are Madagascar (Fig. 10.87), Atlantic forest regions of Eastern Brazil (Fig. 10.88), some regions of Africa and Asian countries. Madagascar or Malagasy may be rightly called “the land of lemurs”.
Here are three to five families of lemurs which include 12-13 living genera and about 26 extant species. The famous mouse lemur, the forked lemur, ringtail lemur, black lemur, hapa lemur, sifaka, indri, aye-aye – all are mostly protected in the Nature Reserves of Malagasy.
Betampona, Tsaratanana, Andringitra, Lokobe, Ankarafantsika, Tsingy of Namoroka, Tsingy of Bemaraha, Tsimanampetsotsa, Andohahela are all famous natural reserves of Malagasy (Madagascar) (Fig. 10.87).
The Atlantic forest region of Eastern Brazil (Fig. 10.88) is especially important for the conservation of primate diversity in global terms, because it includes two highly endangered genera — Brachyteles and Leontopithecus — and a total of 21 primate taxa of which 15 are already endangered.
This forest region consists of a unique series of ecosystems quite distinct from the much more extensive Amazonian forests to the North West.
It stretches much more from the state of Rio Grande do Norte at the easternmost tip of South America south as far as Grande do Sul, the southern-most Brazilian state, and includes the typical Atlantic coastal forests on the low-lying coastal plain.
Species receiving the high priority attention are the Lion-tailed macaque (Macaca silenus) in the tropical rain forest of Western Ghats, South India; the Pileated gibbon or Capped gibbon (Hylobates pileatus) in South East Thailand, Cambodia, and west Mekong; Mountain gorilla (Gorilla gorilla beringei) in Rwanda and Uganda; Goeldi’s monkey in Northern Bolivia, and white nosed Saki Monkey in Brazil. Recently many species of primates are being added to the endangered species list.
In Asia the most endangered primates are the Orangutan (Pongo pygmaeus) in Indonesia and Malaysia, various gibbon species (Hylobates sp.), the Proboscis monkey (Nasalis larvatus) in Borneo, the Douc langur in Vietnam and China, and Simeuleu macaque (Macaca. fascicularis fuscus) in Indonesia.
In Africa the list includes the Pigmy Chimpanzee (Pan paniscus), Red colobus (Colobus preussi), Mandrill (Mandrillus leucophaeus) in Cameroon; the Aye-aye, Indri and Sifakas in Madagascar.
In Latin America the endangered primates are — Brown spider monkey (Ateles fusciceps fusciceps) in Ecuador, Spider monkeys (Ateles belzebuth, A. paniscus) in Colombia, Central American squirrel monkey (Saimiri oerstedi) in western Panama and south western Costa Rica, and Black howler monkey (Alouatta pigra) in Bermudian langing, Belize.
In Central and South America they include the Woolly spider monkey (Lagothrix lagotricha) and Lion tamarins (Leontopithecus chrysopygus) in the Brazilian Atlantic forests, the white uakari, Southern bearded saki, and pied tamarin in Brazilian Amazonia.
Primate conservation is a controversial issue and is being tackled within global strategy on three levels:
(i) Cases involving endangered and vulnerable species,
(ii) Cases where the trade is related to the depletion of the species and
(iii) The third in which the cases where the trade is threatening the species in the National Parks and Sanctuaries or protected areas.
In order to prevent the extinction of the world’s non-human primate species, the primate specialist group of IUCN’s Species Survival Commission (SSC) initiated a global action plan for primate conservation back in 1978.
The original Global Plan is now being working by a series of new regional action plans for Madagascar, Africa, Asia and Central and South America. In 1979, WWF created a special primate programme to provide increased fund for Primate Conservation worldwide.
In 1985, the programme also launched a five-year Global Primate Conservation Campaign to raise $ 1000000 to carry out different projects related to the most endangered primate species.
The campaign proceeds selecting the world-renowned chimpanzee specialist and pioneer in primatology, Dr. Jane Goodall as honorary chairperson and is being directed by Dr. Russell A. Mittermeier, Chairman of IUCN/SSC Primate Specialist Group.
The following measures are emphasized on conservation of the Primates:
i) Surveys to determine the status of unknown primate species.
ii) Conservation of habitat with increased protection in parks with important primate populations.
iii) Establishment of new parks and reserve in critical areas.
iv) Educational campaigns are necessary to increase the awareness of the importance of conservation of primates among local people.
v) Conservation-oriented research programmes are necessary on endangered species to provide basic data on their behaviour and ecology for long term management.
vi) Training programmes of nationals of tropical countries to create skilled personnel’s for the conservation of primate species in future.
vii) A network should be developed among the field researchers and specialists in zoos and other related institutions.
viii) Captive breeding programmes are necessary for those species which are on the brink of extinction and by employing this technique successfully, the extinction of some species in near feature may be avoided.
Cohort (B) Glires:
The cohort includes a large number of forms which were formerly included within the order Rodentia. This cohort is now divided into two orders — Rodentia and Lagomorpha.
Many adaptive parallels in morphology, physiology and behaviour are present between the members of the two orders. It is believed that the two orders have separate lines of evolutionary sojourn, though both arose from primitive insectivores.
The characteristic features of Glires are:
i) The forelimbs are shorter than hind limbs.
ii) The brain is small and small cerebral hemispheres are smooth.
iii) Presence of continually growing incisors.
iv) The temporal fossae are widely open to the orbit.
v) The diastema is present.
5. Classification of Humans:
I. Order 1. Rodentia [L. Rodere = to gnaw] 1,814 species. Rodents:
The order consists of about two thousand species and is numerically the largest of all the mammalian groups. They are global in distribution. Rodents are among the few successful eutherian mammals that thrived well in Australia.
1. Rodents are small, herbivorous and terrestrial animals.
2. The largest living rodent is Capybara (Hydrochoerus) which is about the size of a pig. Beavers, amongst the rodents, are aquatic.
3. The body of the rodents is covered with soft hairs.
4. Eyes are small but pinna is well-developed.
5. The tail is short or long.
6. The forelimbs are smaller than the hind limbs.
7. Limbs are provided with blunt claws.
8. Locomotion is of plant grade type.
9. The brain is small.
10. Cerebral hemispheres are smooth and olfactory lobes are large.
11. Intestine and caecum are large. Jaw muscles are well-developed for the purpose of gnawing.
12. Testes are abdominal or inguinal in position.
13. Teats in females are abdominal. The rate of reproduction is rapid and prolific.
14. The skull is small. Anterior symphyses between the jaws are absent.
15. The pre-maxilla and maxilla are long for housing the incisors and masseter muscle.
16. The jugal is extended to the glenoid cavity in many forms.
17. Zygomatic arch is complete and tympanic bulla is large.
18. The dentition is modified for chewing and gnawing.
19. There are a single pair of incisors in the upper and lower jaw.
20. The incisors are large, sharp and chisel- shaped. Enamels are absent on the posterior surface of the incisors and, as a result, they wear off quickly. As the incisors are provided with persistent open roots, they grow throughout life.
21. Canine and anterior premolars are absent. A gap, called diastema, is present between incisors and cheek teeth.
22. There are two premolars in the upper jaw and one in the lower jaw. The cusps of the premolars and the molars are joined in transverse fashion.
23. The scapula is provided with acromion and sometimes with metacromion process.
24. Clavicles are well-developed.
Suborder (i) Sciuromorpha:
These are of primitive forms. Two upper premolars and one lower premolar are present. Infra-orbital foramen is not enlarged to transmit any part of the masseter muscle.
Examples:
Five striped Palm Squirrel (Funambulus pennanti) and three striped Palm Squirrel (Funambulus palmarum). Both species are found within Indian subcontinent. The three striped Squirrel is more common in eastern, western and southern India.
The five striped Squirrel is common in northern India. Common Giant Flying Squirrel, Petaurista petaurista (forests of the Peninsular south of the Ganges, Western Himalayas), Indian Giant Squirrel, Ratufa indica (forests of Peninsular India, south of the Ganges).
Suborder (ii) Theridomyomorpha:
They form a separate group of squirrel-like rodents. The infraorbital opening is large but does not transmit any part of the masseter muscle. They are of Old World origin and distribution.
Examples:
Anomalurus (West-African flying squirrel), Pedetes (Jumping hare).
Suborder (iii) Castorimorpha:
The sole survivors of the castorimorphs are the beavers. They are aquatic. The feet are webbed and the tail is covered with scale. Large oil glands are present in the anal region.
Example:
Castor (Beaver).
Suborder (iv) Myomorpha:
The infraorbital opening is large. The superficial portion of the masseter muscle passes forward in front of the orbit while the median portion passes through orbit. The hind limbs are longer than forelimbs.
Examples:
Common House Rat, Rattus rattus (Indian peninsula); Brown Rat, Rattus norvegicus; Bandicoot Rat, Bandicota indica; Indian Mole-Rat, Bandicota bengalensis; Indian Field Mouse, Mus booduga; House Mouse, Mus musculus; Indian Bush Rat, Colunda ellioti; Jerboa, Dipus; Jumping mouse, Zapus.
Suborder (v) Caviomorpha:
It includes New World porcupines. Hairs are modified into quills.
Examples:
Cavies, Agouti, Dasyprocta; Chinchilla, Chinchilla.
Chinchilla is a mountain rodent which is domesticated for its valuable fur.
Suborder (vi) Hystricomorpha:
The infraorbital is enlarged to the greatest extent. The median portion of the masseter muscle is enlarged and attached to the jugal arch. In some forms, hairs are modified to form quills. Quills on the body are solid but those on the tail are hollow.
Examples:
Cavia (Guinea-pig), Hystrix (True porcupine of Asia and Africa).
Suborder (vii) Bathyergomorpha:
Only one genus of Bathyergomorpha is found in Africa. They are peculiar rodents and are burrowing in habit. The skin is almost devoid of hair.
Example:
Heterocephalus (Naked rat of Africa).
II. Order 2. Lagomorpha [L. Lagos = hare + morph = form] 63 species. Rabbits, Hares:
The members belonging to the order Lagomorpha were formerly included in a suborder Duplicidentata under order Rodentia because of superficial similarities. In recent years, zoologists consider the similarities with the rodents as the result of convergent evolution.
The fossil forms found in the sediments of Paleocene show that the two groups were quite distinct from each other in their early stage of evolution. They are distributed in Eurasia, Africa, and North America.
1. The members belonging to this group are small herbivorous and gnawing forms.
2. Body is covered with soft hairs.
3. Eyes are large and pinna is long.
4. Hind limbs are larger than the forelimbs. Digits end in claws.
5. The upper lip is provided with a cleft in the middle.
6. The tail is almost vestigial.
7. The brain is small.
8. Masseter muscles are enormously developed but temporal muscles are weak.
9. Testes lie inside scrotum.
10. Nipples are abdominal in position.
11. The skull is small. Figs. 10.89 and 10.90 give an idea of the structural organisation of skull and lower jaw of Oryctolagus, a typical representative of the order.
12. Anterior symphysis of the lower jaws is present.
13. The orbit is small and muscles never invade the infraorbital canal.
14. Lower jaw articulates with squamosal.
15. Squamosal has large glenoid fossa.
16. The maxillae are laterally fenestrated.
17. There are two pairs of incisors in the upper jaw and one pair in the lower jaw.
18. The incisors of upper jaw are of unequal size. A larger pair lie in front and the smaller pair lie behind the front pair. The incisors are chisel-shaped and are covered with thick enamel in the anterior surface and with thin enamel in the ventral surface. They are with persistent pulp.
19. Canines are absent as such, diastema is present.
20. There are three premolars in the upper jaw and two in the lower jaw.
21. Molars are three in number in each jaw.
22. Scapula is with acromian and metacromian processes.
23. Tibia and fibula are fused.
Examples:
Oryctolagus (Rabbit), Lepus nigricollis (Indian Hare), Ochotona roylei (Himalayan Mouse-Hare), Caprologus hispidus (Hispid Hare is found along the foot of the Himalayas from Uttar Pradesh to Assam).
Ochotona is sometimes called pikas or mountain hare or conies, famous for its occurence of cloaca among eutherian mammals.
Cohort (C) Mutica:
The cohort includes a single order Cetacea. These are highly specialised mammals and show complete divergence from their primitive eutherian ancestors. The whales are aquatic in all stages of life and, because of this peculiar adaptation, their affinity with other mammals as well as their phylogenetic history have become obscure.
The characteristic features of the Mutica are:
i) The body is without hairs except a few sensory bristles around the snout in some species.
ii) A sub dermal fat layer, the blubber is present in all cases.
iii) The skin glands are absent.
III. Order 3. Cetacea [L. cetas-whale] 84 species. [Balen whales, Toothed whales, Porpoises]:
Origin:
The available evidence suggests that all groups of cetaceans have developed from one insectivore line which was once aquatic and then rapidly branched (Coffey, 1979). The mysticetes appeared in the Oligocene and radiated in the Miocene, but the different groups of Odontocetes can be traced back to the late Eocene.
A. External Features:
1. The body is large, torpedo-shaped and devoid of hairs. A few sensory bristles are present on the face.
2. The skin is smooth and skin glands are absent.
3. The pinna and nictitating membranes are absent.
4. Nails are absent.
5. Hind limbs are absent.
6. Forelimbs are modified to form the flippers. Table 50 shows the distinction of flipper and paddle.
7. The nasal openings are asymmetrical and located far back on the upper surface of the head which become the blowholes. They are two in whale bone whales and one in the toothed whales.
8. The neck is short.
9. The tail terminates in a horizontal fin that moves up and down.
10. The tail fin, called fluke, is a neomorphic structure. It is stiff and strong but contains no bone.
11. A fleshy dorsal fin, another neomorph, is present on the dorsal side and acts as stabiliser.
B. Internal Features:
(a) Soft parts:
12. A thick subcutaneous layer of fat, called blubber, is present underneath the skin. This acts as heat insulator and reservoir of food.
13. Teeth may be present or absent.
14. Stomach is large and is divided into several chambers.
15. The liver is bilobed.
16. The external nasal aperture can be closed by valves.
17. Epiglottis is tube-like.
18. The diaphragm is thick and obliquely placed.
19. The lungs are highly elastic and extensible.
20. Blood vessels are present all over the body and form networks or Retia mirabilia specially in the thoracic region for storing extra oxygen to be used when under water.
21. Brain is highly developed. Olfactory lobe is reduced.
22. The scrotum is absent and the testes lie inside the abdomen.
23. The penis, when not erected remains curled.
24. The uterus is bicornuate.
25. A single young is born at a time. Gestation period is more than a year.
26. Two mammary glands are located in the inguinal area. The mammary glands are provided with muscles that help secretion. The teats lie in soft pockets.
27. The newborns are usually large and well- formed.
28. In between the vertebrae and tail, long muscles and tendons are present. These are with much myoglobin.
(b) Hard parts:
Fig. 10.91 gives an idea of the skeleton of Balaena.
29. Bones are spongy that helps in buoyancy.
30. In the skull the cranium is dorsoventrally flattened and the facial part is elongated.
31. The jaws are elongated and asymmetrical.
32. The parietals are separated by supraoccipital and inter-parietals.
33. The sagittal suture is absent.
34. Zygomatic process of squamosal is large and stout.
35. Jugals are small.
36. Maxilla is large and extends backwards to overlap the frontals.
37. The premaxilla is long and narrow.
38. Nasals are small. Tympanic bone is large.
39. Tympanic bulla is loosely articulated.
40. Ascending ramus in the lower jaw is absent.
41. The cervical region is very short. Cervical vertebrae are fused to form a bony mass keeping atlas separated. Cervical vertebrae, when remain separated, bear small arches and long transverse processes.
42. Sacral vertebrae are absent.
43. Caudal vertebrae are with chevron bones.
44. Epiphyses remain as separate discs.
45. Zygapophyses are poorly developed or absent.
46. Sternum is much reduced.
47. One pair of ribs articulate with the sternum. Ribs lack heads.
48. Proximal part of forelimb is embedded in the body.
49. Scapula is broad and fan-shaped. Scapular spine is absent. Acromian is curved. Coracoid is compressed. Clavicle is absent.
50. Humerus is short, stout and its head moves freely in the glenoid cavity.
51. The proximal ends of radius and ulna are firmly united with the humerus.
52. Number of digits are increased to more than five (hyperdactyly). Number of phalanges are more than the usual number in 2nd and 3rd digits (hyperphalangy). Fig. 10.92 shows hyperphalangy of the forelimb of dolphin, Globicephala—The vestiges of pelvis remain as a pair of long narrow bones lying parallel to the spinal column a little below the region where chevron bones begin.
52. Vestiges of femur and tibia are often encountered.
